1. Field of the Invention
The present invention relates to a semiconductor-processing apparatus with a self-cleaning device; particularly to a plasma CVD apparatus with a self-cleaning device comprising a cleaning endpoint detection device.
2. Description of the Related Art
CVD apparatuses have been conventionally used to form insulation films such as silicon oxide, silicon nitride, amorphous carbon or polymer containing benzene ring, conductor films such as tungsten silicide, titanium nitride or aluminum alloy, and high-dielectric films containing PZT(PbZr1-xTixO3) or BST(BaxSr1-xTiO3) on a silicon substrate or a glass substrate.
In order to form these films, reaction gas having various compositions or a second reaction gas is supplied into a reactor. These gases cause a chemical reaction by receiving plasma energy, and a desired thin film is formed on a semiconductor substrate. Inside the reactor, a film generated similarly by chemical reaction adheres to its inner wall and a susceptor surface. These adhesive substances are accumulated gradually as a film formation process onto a substrate is repeated; and then, disengaging from the inner wall and the susceptor surface, these adhesive substances sometimes float inside the reactor. This causes impurity contamination that leads to defects in manufactured semiconductor circuits.
In order to remove contaminants adhering to an inner wall of the reactor, in situ Cleaning that cleans its interior while the reactor is in operation is effective. This method is to remove adhesive substances by introducing cleaning gas, which is selected according to a type of adhesive substances, into the reactor to decompose the adhesive substances into gaseous materials. For examples, if silicon oxide or silicon nitride, tungsten or its nitride or silicide adheres, CF4, C2F6, C3F8 or NF3 is used as cleaning gas. Those cleaning gases are activated by making use of a plasma excitation device and electrodes inside the reactor, which are used for film formation, and generate fluorine atoms or fluorine-containing active species. This cleaning using a plasma excitation method is called in-situ “plasma” cleaning. In this case, active species (fluorine radical) of fluorine atoms or fluorine-containing active species decomposes the substances adhering to the inner wall of the reactor and impurities can be removed in the gas phase.
In a plasma CVD apparatus, because a plasma excitation device used for film formation is also used for activation of cleaning gas, large ion bombardment is caused between electrodes by high radio frequency (RF) power applied to the cleaning gas. As a result, the surface of electrodes is damaged; a surface layer comes off to cause impurity contamination. It becomes necessary to replace damaged parts frequently, which increases operation cost.
In order to solve these shortcomings by ion bombardment, remote plasma cleaning was developed. In Japanese Patent Laid-open No. 1998-149989, Japanese Patent Laid-open No. 1998-163185, U.S. Pat. No. 5,788,778, Japanese Patent Laid-open No. 1997-69504, and Japanese Patent Laid-open No. 1993-21531, which are herein incorporated by reference, a method is disclosed in which CF-containing gas or NF3 is used as cleaning gas and excitation of plasma that activates the cleaning gas is performed using microwaves in a second plasma discharge chamber, which is different and separated from the reactor. According to this method, flow-controlled NF3 is introduced into the second plasma discharge chamber, it is dissociated and activated by 2.45 GHz microwaves supplied to the plasma discharge chamber from a microwave oscillator through a waveguide, and fluorine active species are generated. At this time, in order to achieve microwave plasma discharge efficiently, a valve is provided between the second plasma reaction chamber and the reactor to regulate a pressure and the second plasma reaction chamber is maintained at a designated pressure. Generated fluorine radicals are introduced into the reactor in which film formation is performed through a conduit, and decompose and remove adhesive substances on the inner wall of the reactor.
In U.S. Pat. No. 6,736,147, which is herein incorporated by reference, an apparatus in which a remote plasma chamber installed at the top of a reactor and the reactor are connected by aluminum piping and a through-flow type valve is disclosed.
These reactor cleaning processes after film formation are performed by taking sufficient time to remove unnecessary reaction products adhering to the interior of the reactor.
By finding sufficient cleaning time according to a thickness of a film to be formed on a wafer by performing test cleaning in advance, reactor cleaning is performed by storing time data in a self-cleaning device.
In a localized plasma cleaning method in which cleaning gas is activated in a plasma region formed inside a reactor by applying radio-frequency power to electrodes disposed inside the reactor, a method of confirming an endpoint of cleaning the interior of the reactor using plasma emission between the electrodes or voltage/current information of a radio-frequency power supply line is disclosed (for example, in U.S. Pat. No. 5,910,011, U.S. Pat. No. 6,652,710 B2, U.S. Pat. No. 5,160,402, U.S. Pat. No. 5,343,412, U.S. Pat. No. 5,986,747, U.S. Pat. No. 6,368,975 B1, Japanese Patent Laid-open No. 1988-128718, Japanese Patent Laid-open No. 1996-321467, U.S. Pat. No. 6,051,284, Japanese Patent Laid-open No. 1988-14422, Japanese Patent Laid-open No. 1986-145825, Japanese Patent Laid-open No. 1988-89684, Japanese Patent Laid-open No. 1988-14421, Japanese Patent Laid-open No. 1988-244739, Japanese Patent Laid-open No. 1986-247031, Japanese Patent Laid-open No. 1995-169753).
This cleaning endpoint detection is an important item to find whether cleaning of the interior of the reactor has been reliably completed or not; when the cleaning has not been completed within predetermined time, stopping or maintenance of an apparatus is requested based on this information.
In a remote plasma cleaning method using a remote plasma chamber, however, because radio-frequency power is not applied to electrodes inside a reactor, the above-mentioned plasma emission or voltage/current information of a radio-frequency power supply line cannot be obtained.
Consequently, in order to determine whether cleaning of the interior of a reactor has been completed or not, a method using an infrared absorption spectroscope by mounting it at an exit of the reactor, a method using a quadrupole mass spectrometer, etc. are devised. For example, such techniques are disclosed in U.S. Pat. No. 5,879,574, U.S. Pat. No. 6,543,459 B1, U.S. Pat. No. 5,837,094, U.S. Pat. No. 6,660,101 B1, U.S. Pat. No. 6,737,666 B1, U.S. Pat. No. 6,553,335 B2, U.S. Pat. No. 6,366,346 B1, U.S. Pat. No. 6,635,144 B2, Japanese Patent Laid-open No. 1994-224163, U.S. Pat. No. 6,170,492 B1, U.S. Pat. No. 5,902,403, U.S. Pat. No. 6,192,898 B1.
Because a cleaning reaction takes place in the interior of a reactor and further in a region in which reaction products adhering to the interior of vacuum exhaust piping directly connected to the reactor are present, by detecting a cleaning endpoint in a reactor exhaust piping portion, it is possible to estimate a cleaning endpoint inside the reactor to some extent. However, because a state, an amount generated, a temperature, etc. of reaction products adhering to the reactor exhaust piping portion do not necessarily have constant correlation with those inside the reactor, accurate information of the interior of the reactor cannot be detected by the above-mentioned cleaning endpoint detection device installed in the reactor exhaust piping portion. As a result, cleaning time is set rather longer than it should be so as to allow sufficient time, thereby lowering throughput, i.e., substrate processing capacity of an apparatus. Further, there are problems such that, in a method using infrared absorption, due to contamination of a window portion provided in an exhaust piping portion through which infrared rays are transmitted, optical signals obtained are changed; in a method using a quadrupole mass spectrometer, a detection device itself is changed in quality by fluorine-containing active species. It is difficult to perform stable cleaning endpoint detection on a long-term basis, which is required for plants mass-producing semiconductor devices.
It goes without saying that a semiconductor wafer is contaminated inside the reactor by particles and semiconductor device defects are caused if film formation process for a subsequent semiconductor wafer is performed without completing the cleaning.